Positioning systems for antennas on mobile carriers, such as vehicles, aircraft or ships, may attempt to optimally align the antenna with a target, such as a target antenna located on a satellite, during the spatial movement of the mobile carrier. A permanent radio relay link may need to be reliably maintained, even when the carrier is moving rapidly.
2-axes positioning systems may be used in many applications, such as that shown in JP H06-252625 A. Such systems can be used for the independent azimuth and elevation rotation of an antenna. The two axes of such systems may form an orthogonal system, and therefore may allow the antenna to be aligned with any arbitrary point in the three-dimensional space.
If the wireless communication system operates with electromagnetic waves having a linear polarization, a problem that may occur with 2-axes systems is that upon a rotation of the antenna, the polarization planes may also rotate, so that the polarization plane of the target antenna no longer agrees with the polarization plane of the antenna located on the positioning system.
To solve this problem, a third antenna can be introduced for spherical-symmetrical volumes through which the antenna is moved (such as for parabolic antennas), which may allow the antenna to be rotated about the beam axis independently of the azimuth and elevation axes. Such a 3-axes system then may form a complete orthogonal system and allows optimal polarization tracking.
The 3-axes positioning systems for parabolic antennas, however, may not be used for low-profile antennas because independent rotation about the beam axis may not be possible due to the shape of the antenna aperture and the low installation space, or the angular range in which such a rotation is possible may be restricted.
In the case of low-profile antennas that support two orthogonal linear polarizations, polarization tracking may therefore be carried out electronically or electromechanically in the signal processing path, so that no third mechanical axis is needed.
One example of such a 2-axes positioning system according to conventional technologies is shown in FIG. 1. As shown in FIG. 1, 2-axes positioning systems having separate polarization tracking 20 may be used in fuselage or body mounted low-profile antennas on aircraft or vehicles. The antenna systems can be characterized in that the antenna apertures have only a very low height (such as less than 20 cm) so as to minimize the aerodynamic drag to the extent possible. The antenna apertures may be rectangular.
However, for antenna apertures that are not rotation-symmetrical on positioning systems having two axes A and C, upon a rotation of the antenna about the elevation or azimuth axis, the antenna pattern may change spatially in relation to the target antenna and the surroundings thereof because the antenna pattern is not rotation-symmetrical.
Geographic skew may therefore arise in the communication with satellites, such as in applications on mobile carriers such as aircraft, which can cover large geographic distances.
Geographic skew may be due to the azimuth axis of the antenna aperture being located in the aircraft plane in a 2-axes positioning system. The aircraft plane may be a tangent plane to the earth's surface. If the aircraft position and the satellite position are not located on the same geographic longitude, the antenna aperture, when it is directed at the satellite, may be rotated with respect to the plane of the Clarke orbit by a certain angle, which may depend on the geographic longitude.
Because the width of the main beam of low-profile antenna apertures can continue to increase as the rotation about the beam axis increases (proceeding from the normal azimuth position), the power spectral density in the transmission operation of the antenna in the fixed satellite service (FSS) may need to be successively reduced to ensure regulatory compliant operation.
The worst case in the FSS may occur when the mobile carrier is below or in the vicinity of the equator. The main beam may then have the maximum width with respect to the tangent to the geostationary orbit at the location of the target satellite, and impermissible irradiation of neighboring satellites may occur.
Problems may also arise in reception because the signals of neighboring satellites may be received together with the signals of the target satellite, and substantially no discrimination may take place via the antenna pattern. The signals of the neighboring satellites then may act as disturbing signals (e.g., noise), which are superimposed on the wanted signal and corrupt the wanted signal. The receivable data rate may decreases in this case.
The reduction of power spectral density of the transmitted signal and the interference of neighboring satellites in the received signal may mean that low-profile antennas cannot to be operated on 2-axes positioning systems in the vicinity of the equator in the FSS, or may only operate with a considerable loss of performance.